Electrostatic microwave measuring system



SEARCH *RoomI ELECTROSTATIC MICROWAVE MEASURING SYSTEM Original FiledJune l1, 1945 3 Sheets-Sheet 1 70 '-M/c,ewv4vf i P0555 w/n/f a/af iGMM/:me *LW/T5@ IN V EN TOR.

CROSS QFFFPFNPF June'l; 1948. L. E. NORTON 2,442,314

4 ELECTROSTATIC MCROWAVE MEASURING SYSTEM Original Filed June 11, 19.453 Sheets-Sheet 2 ngz PULSE KE Y/NG C/AECU/ T 7'0 LIM/751 57 00m/7%INVENTOR.

ZowellE/brm ATTORNEY SEARCH Pw SEARCH' `une 1', 1948. L. E. NORTONELECTROSTATIG MICRWAVE MEASURING SYSTEM 3 Sheets-Sheet:

Orignl Filed June ll, 1945 INVENToR. L awellE/rn BY Q@ fl A 77'0/PA/Ey*4b amel K and assigned to the same assignee as the methods of and meansfor Patented June 1, 1948 ELECTROSTATIC MICROWAVE MEASURING SYSTEMLowell E. Norton, Princeton Junction, N. J., assignor to RadioCorporation of America, a corporation of Delaware Original applicationJune 11, 1945, Serial'No. 598,739. Divided and this application April30, 1946, Serial No. 665,958

This application is adivision of applicant's copending -applicationSerial No. 598,739, filed June 11, 1945, entitled Microwave measuringapparatus;

application.

This invention relates generally to improved measuringrrgcrowave andmore.nanieulerlnterfs and methods,-mlmeasurirgmira., averti-1g, iS-',sion through waveguidespr coaxialV transmis` on lnesgbmemploying thedisplacement of a portion` of the outer wall of the transmission systemdue to stresses induced therein by the electric fields within thesystem.

Typical systems and methods will be described hereinafter by reference ttheir application to waveguide transmission systems. However, the sameprinciples may be applied to coaxial transmission systems. If a portionof one of the wide faces of 'a waveguide is removed, and a exibleconductive diaphragm issubstituted therefor, the

lstresses induced in the diaphragm in response to the electric fieldsdue to microwave propagation through the waveguide will providemechanical displa-cement of the diaphragm as a function of the strengthof the electric fields. An increase in the electric' ield strength willtend to displace the diaphragm Closer to the opposite waveguide wallwhile a lesser eld strength will exert correspondingly less force uponthe flexible diaphragm. If the diaphragm is highly resilient, and a goodelectrical conductor, its inherently high Q will provide measurablemechanical displacement with negligible absorption ofmicrowave energy.The eiiect upon the diaphragm of the corresponding magnetic elds withinthe waveguide will tend to reduce said displacement to a slight extent,but the resultant of the displacements due to the electric and themagnetic elds may be usefully employed for measurements of the microwaveenergy propagated through the guide.

The mechanical displacement of the flexible conductive diaphragm may beemployed to geneerate electric potentials as a function of the pressureapplied by the diaphragm to a piezo crystal, or alternately, thediaphragm may comprise one electrode of a variable capacitor whereon thecapacitance or the potential upon the charged capacitor may be employedto actuate an indicator or a control circuit.

Among the objects of the invention are to provide improved methods ofand means for measuring microwave energy. Another object of the instanttolaims. (01.171495) invention is to provide improved methods of andmeans for measuring microwave power propagated through a waveguidesystem, A further object of the invention is to provide improved methodsof and means for measuring microwave energy propagated through a coaxialtransmission system. An additional object of the invention is toprovideimproved'methods of and means for measuring microwave propagationthrough a wave transmission system by employing the mechanicaldisplacement of a flexible conductive element forming a portion of saidsystem and responsive to the microwave fields therein. Another object isto provide improved methods of and means Vfor measuring microwave energypropagated through a waveguide wherein the mechanical displacement of aportion of the waveguide wall provides mechanical deformation of a piezocrystal for generating electric potentials characteristic of themicrowave transmission.

An additional object is to provide improved methods of and means formeasuring microwave transmission through a waveguide wherein a liexibleconductive element forming a portion of the waveguide walls comprisesone electrode of a variable capacitor, and wherein the eectivecapacitance, or the charge upon said capacitor, is al measure ofthemicrowave energy propagated through said guide. A still furtherobject of the invention is to provide an improved method of and meansfor. detecting microwave energy propagated through a waveguide includingmeans for modulatingsaid microwave energy as a function of the energydetected by a mechanical element responsive to the microwave field. Anadditional object is to provide improved methods of and means formeasuring` microwave transmission through a waveguide or coaxialtransmission system including a novel indicator coupling circuit forminimizing noise signal components of the measured microwave energy.

The invention will be described in greater detail by reference to theaccompanying drawings ofwhich Figure 1 is a schematic diagramillustrating the mechanical displacement of a flexible conductiveelement subjected to varying electric fields, Figure'Z is a perspectiveview of a typical waveguide including a iiexible conductive diaphragminserted in one of the wide faces thereof, Figure 3 is a transverselycross-sectional, partially schematic diagram of a rst embodiment of theinvention, Figure 4 is a longitudinally cross-sectional view of arstmodiiication of sai-d first embodiment ofthe invention, Figures 5 and6 are schematic circuit diagrams of typical coupling circuits which maybe employed with any CROSS RFRECE of the embodiments of the inventiondescribed herein, Figure 7 is a longitudinally cross-sectional,partially schematic view of a second modication of said rst embodimentofthe invention, Figure 8 is a longitudinally cross-sectional, partiallyschematic view of a third modification of said rst embodiment of theinvention, Figure. 9 is a. transversely cross-sectional view taken alongthe section line IX-IX of a second embodiment v of the invention, Figure10 is a plan view of said second embodiment of the invention, Figures l1and, 12 are schematic diagrams 'orcircuitsy ernploying said secondembodiment of the invention', Figures 13 and 14 are longitudinally'cross-sectional, partially schematicv diagrams of a thirdembodiment ofthe invention, Figure 15 'is'a transversely cross-sectional elevationalView of a fourth embodiment of the invention adapted to coaxialtransmission line systems, and Figure 16 is a bottom view taken alongthe line XVI-Zivi of saidiourth embodiment of the invention.Similarr'eferen'c'e characters 'are applied to `similar elementsthroughout the drawings. Y

, Referring to Figure l, one. of the Vwide-'faces I'of a waveguide isnormally spaceda distance s .from the opposite wide face ,2 thereofvwhich 'includes a exible conductive diaphragm of the type describedherein. If modulated microwaves are propagated between the waveguideconduct'ors .l andy 2,vv the forces acting on the 'flexible diaphragmforming-a portion of the waveguide conductor 2. will be proportional tothe resultant of the stresses induced therein by the microwave lelectricand 'magnetic iields. At microwave-frequencies the electrodes I and 2'will have both lwherein C-i's the capacitance and `V is the voltagebetween the conductor-'sof the'ap'acitor,

.ffhl and is maintained inweguilibrium by aholding forcer'lthe-resultant stored energy is fs v"si Tas L1. w,=mo(js v) ergs v1(2)VSincel one -statvolt='3`00. volts "and---on'e -statfarad='1/9 10l1farads, if C and 'V are expressed in farads and volts; respectively,'then Atypical application oi Ythe invention'toa rectangular waveguidesystem, illustrated inoFigure 2, includes a flexiblev conductivediaphragm 3 107 dynes waveguide system T. Forthe sake ofillustration,

trode lis displaced'a distance dS' the conductive diaphragm 3 extendsthe full width of thewaveguide face, and also extends a distance b alongsaid waveguide face in the direction of wave propagation. For purposesof computation, it may be that. th guided microwave propagation'albngthe waveguide is in the X and .-X directions and that the electricvector is in the ZY plane. For most types of operation -it is desirablethat lthe :conductive diaphragm 3 have high mechanical Q and relativelyhigh deiiection sensitivity at the microxwave modulationoi'-irderruption rate. lllhe microwave modulation "o'r interruption maybe accomsetinto thc upper waveguide face E of atypical i pli'shed by'modul-ating the microwave source directly,. 'orl byinterrupting themicrowave transmissionby means of a. suitable shutter or other 4devicedisposed between the microwave source and the, flexible diaphragm.

Employing the XYZ coordinate system of Figure 2, the electric intensityis represented by E==Eof (X, Y, Z, 15)-, where -t is-'tlf'r'e-timef-irr4s'econds. Y

In conven" "initial gidedf'inicrovvave propereat'o'n utilizing highconductivity waveguide 'wail conductors parallel to the XY plane,and-disregarding the dependence of the 'voltageE-upon--the Y aX-S, u

1ra-'Eu sinistrate) 69 The electric ic' acting on lthe parallel upperand lower waveguide walls on a'unitfarea of said walls is "F=KE'* (7')so that Y' AFfn-KEMA-sin-ratigkr) `la) where AFi's the torce acting onan incremental wall area-AA (which-is'sma-ll with respect tothewavelength scale) For-a waveguide wall of lunity width along the Y axisand extending from,-X=a to X=(a\'b) along the X axis, theelectricforceacting' on the opposite waveguide-walls per'unit width and per uni-tlength for anylength-lip per cycle of thepropagated radiation 'is SEARCHeww r 1 mmf... ..1.1

am PE1-J2) a sin udg-2a o lr-cos a )dy- E2 a 1H- U2 a 'E2 al1/.ra @aThereforerthe force per unit area where E is a lfunction of all fourvariables (X, Y, Z, t) in guided vwave propagation is As statedheretofore the forces due to the electric fields of the propagatedmicrowaves are always attracting. A similar calculation for the forcesexerted between the waveguide walls due to the currents flowing in saidwalls indicates that such forces due to the magnetic field areapproximately one-sixth as great as the forces due to the electricfield. Since the sign of the forces due to the magnetic field areopposite in sign to those due to the electric field, the resultant forceexerted between the opposite waveguide walls is about ve-sixths of theforce due to the.l electric eld alone. Y

Formula 14 showsthat the force Fo for a constant field Eo isvindependentcfthe microwave frequency. However, the field Eo is notindependent of frequency. In order tomake the electric field Enindependent of .frequency and therefore to make Fo completelyindependent of frequency, the conductive diaphragm may be employed toform a portion of the outer conductor of aconcentric transmission linewhich is substituted .for the waveguide system described heretofore.Such a. concentric transmission line should be proportioned so that theinner diameter ofthe outer conductor of the, line is very smallascompared to a wavelength of the transmitted microwave energy.

As has been shown heretofore the'forces and potentials developed upontwo parallel electrodes of a capacitor, of which the flexible conductivesystem. Forces acting upon the crystal electrode diaphragm is oneelectrode, may be employed for measuring the magnitude of the propagatedmicrowave energy by measuring the variation in Acapacitance 'on thevariation in thecharge upon said capacitance. However, the forces actingupon the flexible conductive diaphragm also may be employedto providemechanical deformation of "a piezo crystal which is maintained incontact .with the flexible conductive diaphragm. For example. a piezocrystal having conductive electrodes coated thereon byelectro-deposition may be set into a suitable aperture in one of thewide faces of a waveguide or coaxial transmission system whereby theconductive coating on the lower side of the crystal comprises the exiblediaphragm described heretofore.` A relatively heavy electrode may beheld in contact with the opposite crystal electrode by suitable springtensionl or other means which effectively prevents movement of thecrystal with respect to the waveguide forming a portion of the waveguidewalls will provide mechanical deformation of the crystal, thusgenerating varying potentials betweenthe crystaly electrodes v whichwill be characteristic of the magnitude of the propagated microwaves..If the: crystal is resonated to the microwave modulation frequency, thesensitivity of the system. may be greatly improved.

Formula 5 is the force acting on the lowerelectrode surface of thecrystal; The potentials appearing across the crystal electrodes causedby' the disturbing force E may be determined as fol-- lows. For aparallel platecapacitor separated by' dielectric material of dielectricconstant K. a, spacing S1, and an electrode area A CFL-l statfarads or IISY cm. and V=1 Ivolt (corresponding to an venergy level in a standardx-band waveguide of .001'. watt), from Equation 5 j' v l F .0884X 10-2X101:..442X 10-6 dynes (17) Also, since A=1 cm?, the force per cin.2 alsois F1=A42 106 dynes/cm.2 .(18) For piezo electric materials charges andexerted forces are related by expressions of the form i Y' Q=K1f v (19)For quartz, if f is in dynes, Ki=6.4 10*8,`then Qis in statcoulombs. f l1' Assuming that the piezo-electric crystal and its upper electrode havethe same dielectric area as the lower electrode, and that'K for quartzislapproximately equal to 5, 5" i :i:

'C'-Z%=; statfarads where K=5,'A 1 cm.2 and Si=l cm.`` "'For quartz fromFormulas 18 andv 19 'i Y Q'i-:SAXl-BXAiZX 106= l 2.83)(-10-14statcoulombs '(21) n It is' now necessary to find the potentialvappearing across a condenser of. capacity C and y In a. condenser, C,I, E; and t'are related by theexpression .Therefora from Equations25A,.v21,.20 the potential appearing across the condenser is that E=.21310-" volts. In either case the potential developed across the crystalelectrodes is so that at approximately room temperature, and

selecting a value of R for the grid resistor 31 equal f' to Rg, the gridcathode resistance, or 107 ohms gnw Y (31) However, if R is equal to Rg/which is approximately equal to the resonant resistance of thecrystal *iY 9.1145 l (32) A 64 v thereby indicating that the circuit congurati-Onof Figure 6 is preferable to that of Figure 5.

Figure 'I shows an electromechanical means for pulsing continuousmicrowave signals derived from the generator 23 and propagated throughthewaveguide 1.v An aperturey device 5l interposed in the waveguide 'Iintermediate the waveguide 23 and the crystal 9 cooperates with atransversely movable shutter 53 which is coupled through a linkage orlever mechanism 55 to, for example, the moving coil structure 51 of adynamic motor mechanism 59. The ield structure 6| of the motormechanismf59 includes a iield winding. E3 whichvis connected to a sourceof eld current such, for example, as a battery 55. The moving coil 51 isactuated by current pulses derived from the pulse keying circuit 25which is responsive tothe output of thelimiter 31 as describedheretofore. ?ulses derived from the keying circuit-.thereby actuate themoving coil 51, and the motion thereof is transmitted through the levermechanism 55 to move the shutter 53 in a transverse direction across theaperture device 5I -for interrupting the propagation along the waveguideof the continuous microwaves.

Figure 8 shows means whereby the continuous microwaves derivedfrom thegenerator 23 may be interrupted by an ionic discharge across a spark gapdevice B1 forming an aperture in the waveguide 1 intermediate'thegenerator and the v crystal. The spark gap device B1 may be a resonantor non-resonant aperture, of any type known inthe microwave art, whichis actuated by pulses derived bythe pulse keying circuit 25. Pulsesderived from the keying circuit thus generate a spark dischargek acrossthe spark gap of the device 61. The. spark discharge eiectivelyshort-circuits the waveguide 1, thus preventing microwave propagationalong the guide to 'the crystal and load for the durationv of the sparkdischarge. v

Figures 9 and 10 show the structure of a typi cal variable capacitordevice, one electrode 69 of which is -a flexible conductive screencovering the aperture in the wide upper wall 5 of the rectangularwaveguide 1. The conductive screen may comprise, for example, a coppermembrane hav'- inga 4-Inesh of 1000 conductors per linear inch and athickness of the order of 1 mil or less. It may be produced byphoto-deposition or other known means and suitably clamped in asupporting bezel. A- typical method Iof making such a fine screen isdisclosed in the copending U. S. application of Harold B. Law, SerialNo. 531,008, filed April 14, 1944, now abandoned. An insulatr ingblock1I mounted on the upper waveguide wall 5 supports a second capacitorelectrode 13 in` a position which is parallel to and closely adjacent tothe flexible conductive diaphragm' 69. Thus deformation of the diaphragm69- in response to the microwave elds of the microwave energy propagatedthrough the waveguide 1 varies the eiective capacitance C between thecapacitor electrodes 69 and 13.

The circuit of Figure 11 shows the'means'in which the variablecapacitance C provided by the electrodes 69 and 13 may be coupled to anindicator circuit, not shown, to indicate the variation in theelectrical charge upon said capac-fA itcr in response to variations inthe capacitance thereof. The variable capacitor C is charged by means ofa circuit-including a charging battery 15 and a series charging resistor11. 'I'he potential lupon the charged capacitorC is coupled through agrid coupling capacitor 19 to the control electrode of an'input couplingtube 8l A grid bias battery 83 serially-connected with a grid resistor85 is connected across .the control grid-cathode circuit'of the tube.The anode and screen circuits of the tube 8l are similar to thosedescribed heretofore with respectcto the circuits of Figures 5 and 6.Since there is a constant charging voltage circuit for capacitor Cthrough the resistor 11, and since the time constant of the elements 11and C is long at the modulation.

frequency, the charge on C remains constant during the modulation cycle,sc that the variation in capacitance -due to the microwave elds withinthe waveguide provides a varying potential' upon the charged capacitor Cwhich 4is ap'- plied to the control gridA circuit ofthe tube 8l;Thispotential varies at the microwave keying frequency and may beemployed to control' an amplier and indicator circuit in the same manneras described heretofore.

The variable capacitance provided by the lexe ible conductive screen 69'ci' the device of Figures 9 and 10 also may be employed tomodulate thefrequency of an oscillator circuit whereby the oscillator frequencyshift due to the variations in capacitance are a measure of thecharacter-y istics of the microwaves. propagated through thewaveguide 1. i Figure 12 shows the capacitance as the tuning element ofa' tank circuit which includes a tapped inductor 81 connected in aconventional Hartley oscillator circuit including a triode tube 39. Theanode circuit of the oscillator tube 89 is con-- nected through acoupling capacitor 9| to a frequency discriminator 93 which provides acurrent or voltage proportional to the oscillator frequencyvshift due tothe capacitance variations. The output of the discriminator. 93 iscoupled through an amplifier 33 to an indicator 35 in the same manner asdescribed heretofore.

Figure 13 shows a coaxial line type of oscillator tank circuit for aHartley oscillator wherein the capacitive electrode 13 is closelycoupled-to' the rFlexible conductive screen 69 which covers the aperturein the upper wall 5 of the waveguide mentarily threaded inner lineconductor 95 toA adjust the spacing of the capacitive electrodes 69 and13. Coupling from the anode of the oscillator tube 89 to the centerconductor S5 of The ends of the coaxial conductors Il' thecoaxialtank-circui-tis accomplished fby'means. of a conductor' FUI.''passing through: an; insulated bushing |-Il3`disposedi intermediatethalinei conductors 95' and' 921' a. short.' distance. from the;capacitive electrode 1531.

An anode coupling; capacitor It-is.' connected sexies with theconductorll to;isolai:eA anode voltage from the tanli` circuit.. 'l'heAgrid off. the oscillator tube 82T isconnectedv directly.'- toi theouter` conductor 9T" ori the tank; circuit. The cathode of.' theoscillator tube 8S is connected through aseries circuity comprisinganiv'ndu'ctor |015 and aparalleI-connectedf cathode resisizor lIlSLandbypass capacitor iI-I; to'y the outer tant:A circuitv conductor 9T.VOutput. signalsare derived bymeansz of' a coupling. loon `I I3 coupled'to the cathode inductor |1111 and'connected through: anoutputcoaxialline |J|`5fto the,output-circuit'.Y The ontput'- signalpotential be' of the order ot one-tenth of theoscillator voltagedeveloped' across the tank' circuit;

'I'Ihe circuit congurati'on oi Figure: l has the disedvantagey that theoutput-potential; mustibe: rather low order to 1 prevent 'excessiveshmlt or: parallelil loading of the oscillator'c'rrcuit to pro'-videecient control thereof by the'smallv'ariabief' capacitorcomprisingthe electrodes: E8'- and'. 13; The circuit of Figure 14 showsan improvedl output.' coupling circuit having extremelyy highresistance. which permits theanod'eof the ocsillator' tube: 89 to be:coupled;` to: the: control el'ectroci'e-v of. an. ampliiier tube:I'|-'|= which. mayA be either'a triode.x or' apentode; and'whichisconnected; as. ai cathode.' follower: The anode of the oscillator tube89! coupled throught-a. coupling capacitor I; I3- to the control grid;of the cathodefcllower tH. extremely high value grid re sister' tZI isconnected from the:` control, grid of the cathode follower Irl-T tocanintermediatepoint on: a seriesA cathode resistor |:23- which isseriallybconnected' through a;k coaxial line induc-A tor' 25 toground..` An intermediate point |21 on: theA inner conductorofthecathode) coaxial line: inductor |25." is coupled through` amoutputcoaxiale-line |29 ton any desired: low impedance output circuit.'

The advantage of the cathode follower-'circuit thus described isthat.:its: effective inputV resistance may he: made oi the order of severalthou sand: megohms.' by proper' selection of' gridzand cathode circuitparameters, while the output ciri cuti may 'be Vdi? relatively: low'impedance. Sat.'r SfUtOll/ output signal level is. thus. provided' withVmaximum useful signal-to-noise signal' voltage ratio. Ii desired thesametype of cathode. follower circuit may' ha substituted` for thecircuits'- oi Figuresand 6 for directly.` coupling the ampliex: andindica-tory circuits to. the crystal dei vices described heretofore-Figures 15 and." 1d show the: application of, the variable: capacitivefeatures of the.- invention to coaxial line technique which provides asubstantiallyl frequency independent 'embodiment of. the. inventiom. A.portion of the outer conductor I-3-I K oi.- a coaxial: line |33,Yhaving.A aninner conductor |35, 'is removed to pernnt: the' insertion0fv the flexiblev munductive.v diaphragm, or screen |31L A bracket |39,to' which an insulating block I4! is mounted', supports a fixedconductive electrode |431 in. close proximity tov the exible conductivediaphragmv |31. This device may be coupled to measuring circuits whichpermit measurements 'ofreither the,- electrical potential, or thefrequency shiftin an oscillator circuit,- due tothe capacitive.Variations resulting from deformation. of, the;

12 flexible diaphragm |31 inV response Y tothe pulsed. microwave neld inthe coaxial lines As. explainedheretofore, in order that the coaxialline'embodiment yof the invention may be'substanti-al-ly vfre-- quencyindependent, the radius. of the inner surgenerating potentials bymeansof a piezo crystal having said element for one'of its electrodes, or

2a. by means of a. variable capacitor of. which the element. comprises'one'. electrode; Also several coirpling circuits are disclosed' 'forprovi'dingem'ci'ent' electrical couplmg tothe eld'responsive devices.

I; .Apparatus for measuring mcrowaVespropa-igated throughs waveguide'.transmission system'v including a .conductive diaphragmforming aportionof' the V.transverse wall' of saidwaveguide,v said' diaphragm being'subjected" tovariabl'e mechanical" displacement due substantially onlytoI the varying electrical' field' ofsaid microwaves-a' nxed conductive;electrode` disposed'ad'jacent said` diaphragm" and" forming a' variableimpedance' therewith, and' means responsive to vsaid variable impedancefor indicatingdthe"magnitude of sai-dpropagated; microwaves.

2; Apparatus iormeasuring'microwavespropa'- gated through. a waveguidetransmission systemA includinga conductive diaphragm forming apor- 40tion of the transverse wall of' said waveguide;4 said diaphragm' beingsubjected to variable mechanicalA displacement due substantially only`tcr the' varying electrical field of said microwaves, a'flxediconductive electrode disposed 'adjacent' saiddiaphragm and formingavariable'impedance' therewith, a generator having its frequencycontrolled by said variable. impedance, means' for measuring-' saidfrequency and means for indicating the' magnitude of said' propagated"microwaves as' ace function of said measured' frequency.

' 3.. Apparatus'for measuring microwaves propagated through a waveguidetransmission system includingia conductive diaphragm forming a portionof the transverse wall of. said waveguide, said diaphragmbeingsubjected' to variable mechanical. displacement due substantially onlyto the varying electrical eld. of said microwaves-a iixed conductiveelectrode disposed adjacent said diaphragm and forming a variableimpedance therewith, substantially constant potential means chargingsaidvariable impedance, means for. detecting the variations in the chargeson said impedance. and means for indicating the magnitude of saidpropagated microwaves as a unctiomof said charge. variations..

4. Apparatus for measuring modulated micro waves propagated through. awaveguide trans mission system including a conductive diaphragm forminga portion of thetransverse wall of said waveguide, said diaphragm beingsubjected 'to'l variable mechanical displacement due substan' tiallyonly to the varying electrical eld of said microwaves, a fixedconductive electrode disposed adjacent said diaphragm and forming a.variable impedance therewith, means for deriving signals.

tially only to the varying electrical eld of said microwaves, a xedconductive electrode disposed adjacent;l said diaphragm and forming a,variable impedance therewith, means for deriving signals in response tosaid impedance variations, means disposed in said waveguide system forkeying said microwaves, a feedback circuit responsive to said signalsfor controlling aid microwave keying means, and means responsive to saidsignals for lo indicating the magnitude of said microwaves.

LOWELL E. NORTON.

